Increasing awareness of the effects of vehicle evaporative and exhaust emissions has resulted in regulations at both state and federal levels to control these emissions. In particular, on-board diagnostic regulations require that certain emission related systems on the vehicle be monitored, and that a vehicle operator be notified if the system is not functioning in a predetermined manner.
One example of an emission related system is a fuel system, which includes a fuel tank for storing a fuel. Vapors from the fuel collect within the fuel tank. Occasionally, the fuel tank may develop a leak due to a hole, such as from a sharp object puncturing the fuel tank. Therefore, vapors present within the tank may inadvertently escape from the fuel tank and into the atmosphere. A primary component of the fuel vapor is hydrocarbon, which is known to have a detrimental effect on air quality. Currently, on-board diagnostic regulations require that a diagnostic small leak test and a very small leak test be performed periodically while the vehicle is operational, to detect a leak in the fuel tank. If a leak is detected by the diagnostic test, the vehicle operator is notified.
Various test procedures are used to detect a small leak or very small leak in the fuel tank. In one example, an overall slope of a vacuum decay rate is determined by measuring an induced vacuum within the fuel tank at a beginning of a test and the vacuum at the end of the test. If the overall slope does not meet a predetermined criteria, there may be a leak in the fuel tank. One example of a predetermined criteria is a maximum slope threshold. However, a shortfall of the overall slope test procedure is that it does not account for conditions when the vacuum decay rate is not decreasing in a predictable manner, due to a typical operating condition of the vehicle. For example, fuel slosh, or turbulence of the fuel within the fuel tank occurs when the vehicle undergoes a series of sudden movements. Fuel slosh may affect the actual vacuum decay rate positively or negatively. Consequently, a driver occupant of the vehicle could either be erroneously notified of a malfunction, or fail to be notified, depending on the circumstance. Thus, there is a need in the art for a reliable method of validating a diagnostic leak detection test that is not sensitive to fluctuations in vehicle operating conditions.